1. Field of the Invention
A direction finding interferometer determines direction by ascertaining phase differences in signals received by multiple antennas. Frequent recalibration of such devices is necessary because the transmission characteristics of the various transmission lines and components can vary due to, for example, thermal effects and equipment aging. Recalibration determines the phase differences between the receiver channels following the antenna elements.
Recalibration is especially problematic in long-baseline interferometers, in which the antenna elements are spaced far apart. In such an interferometer it is not feasible to control precisely the electrical lengths of the transmission lines that are necessary for recalibration of the interferometer. What is needed is a recalibration system which permits frequent recalibration and is independent of transmission line length.
2. Description of the Prior Art
In general, recalibration systems for direction finding interferometers in the prior art require external calibration means such as beacons and associated instrumentation, which are generally not feasible for frequent recalibration. Alternative systems require stabilization of the electrical lengths of the transmission lines.
For example, M. Nollet et al., "Advanced VHF Interferometer Spacecraft Tracking System," Electrical Communication, Vol. 49, No. 3, 1974, discloses a system for reducing errors due to the uncertainty of the electrical lengths of the transmission lines. Bidirectional transmission of two calibration signals at different frequencies through the system is used to determine the electrical lengths of the transmission lines. However, precise calibration is impaired by indeterminable frequency-related phase shifts through the different network paths.